1. Field of the Invention
This invention relates to absorbent compositions, methods for producing the absorbent compositions and absorbent products. More specifically, the present invention relates to water absorbents having a structure in which a water absorptive resin contains a built-in micro-filler so as to improve the surface area, absorption speed (especially diffusive absorption speed) and the initial amount of absorption under applied pressure. The present invention also relates to processes for manufacturing these absorbents and absorbent products using the water absorbents.
2. Background Information
Absorption speed of a water absorptive resin depends upon the surface area of the resin. That is, in a particle type water absorptive resin having a constant mass, the absorption speed of the resin becomes slower as the particle size becomes greater because the area that contacts the water becomes smaller. Conversely, the absorption speed becomes faster as the particle size becomes smaller because the contact surface area increases accordingly. However, if the particle size is too small, a phenomenon happens in which particles of the water absorptive resin cohere with each other via water (a phenomenon called "undissolved lumping") upon contact with water. Thus, the apparent absorption speed becomes slower.
In order to improve the absorption speed of a water absorptive resin, methods such as those described in 1 to 6 below have been proposed:
1 Adding surface active agents or a water soluble polymer on the surface of a small particle type water absorptive resin to prevent "undissolved lumping" (Japanese Patent Application Public Disclosure No. 2-30333);
2 Producing a porous resin by adding a low boiling point volatile solvent to a polymerization solution to be used in the manufacturing process of a water absorptive resin, and vaporizing the volatile solvent by polymerization heat to obtain the porous resin (Japanese Laid-Open Patent Application No. 59-18712);
3 Producing a foam type resin by adding a crosslinking agent and a pyrolytic foaming agent to a mixture of a water absorptive resin having carboxyl groups and a polyolefin resin having glycidyl groups so as to generate foam by heat to obtain the foam type resin (Japanese Laid-Open Patent Application No. 63-251437);
4 Producing a porous water absorptive resin by dispersing a foaming agent comprising an azo compound having amino groups in an aqueous monomer solution containing an unsaturated monomer and a crosslinking agent so as to polymerize the monomer with the azo compound to obtain the porous water absorptive resin (Japanese PCT Laid-Open Publication No. WO 96-17884);
5 Granulating particles using water or a heat melting resin binder; and
6 Covering the surface of a water absorptive resin particle using porous inorganic particles, the covered particles weighing approximately some multiple of ten percent in relation to the total water absorptive resin particles.
However, the above-indicated methods are not completely satisfactory in terms of production and quality for the following reasons.
The water absorptive resin obtained by using a method based on 1 has a problem when it is applied to a disposable diaper as an absorbent that is mixed with fibrous material such as pulp. The small resin particles tend to separate from the fibrous material.
Also, the water absorptive resin obtained by a method based on 1 can generate problems such as a decrease in the ability of fine particles to flow because the particles are treated with a surface active agent or water soluble polymer. Deterioration of the operation environment can also occur because the fine particles can become the cause of dust generation.
According to the methods based on 2 for making porous resin, while some improvement in the absorption speed is observed, a special explosion proof facility is required because a low boiling point volatile solvent is used.
According to the methods based on 3 in which a pyrolytic foaming agent is used, it is difficult to obtain a constant absorption speed and absorption performance. The difficulties arise because the material used, i.e., a water absorptive resin and a polyolefin resin, has no flexibility and discharges gases once the gas pressure reaches a certain level producing a large diameter non-uniform foam.
According to the methods based on 4 for producing porous resin, problems such as a decline in absorption performance are caused. This decline is due to a decrease in the molecular weight of the water absorptive resin and an increase in the amount of water soluble components. The decrease in molecular weight happens because a radical is concurrently produced when the azo compound, which has amino groups, decomposes and generates nitrogen gas.
According to the methods based on 5 in which particles are granulated using a binder, the improvement in absorption speed becomes insufficient when the adhesion force of the binder is increased. On the contrary, if the adhesion force of the binder is decreased, the mechanical strength of the granules are weaken and can be destroyed and return to powder by way of a screw feeder or mechanical shear during a powder transfer process.
The methods based on 6 in which the surface of a part of the water absorptive resin is covered by porous inorganic particles also have problems. For the methods based on 6, instant absorption speed at an initial absorption stage is somewhat improved due to an increase in the apparent surface area of the resin particle. However, no improvement is observed in terms of diffusive absorption speed inside the particle or an initial absorption amount under applied pressure. This is because no effect is obtained on the permeation of liquid inside the resin. Moreover, problems such as a lowering in absorption efficiency of a resulting composition are caused since the method requires using a large amount of inorganic particles.
The above mentioned methods 2 to 6 are means to improve instant absorbability at an initial stage of absorption by increasing the surface area of resin particles. The methods have little effect on solving gel blocking. In other words, the above-mentioned "undissolved lumping" occurs when a large volume of water absorptive resin particles is present. Also, these methods have little effect on improving the diffusive absorption speed or the initial amount of absorption under applied pressure since the permeation of liquid to be absorbed inside the resin particle is not fully improved.
Also, when blood is the liquid to be absorbed, the above mentioned methods 1 to 5 do not give completely satisfactory results. This lack of satisfactory results is because the viscosity of blood is relatively high and contains high molecular organic components such as blood corpuscles, hemoglobin, cytoplasm and proteins. Accordingly, a water absorptive resin that provides the desired blood absorption efficiency, both the desired amount of retention and the absorption speed, and is suitable for blood absorbent products such as menstrual products have been long awaited. In order to improve absorption performance of a water absorptive resin for blood, methods such as following 7 to 9 have been proposed:
7 Adding a salt of inorganic acids or organic acids to a water absorptive resin (Japanese Laid-Open Patent Application No. 58-501107);
8 Using a potassium salt or a lithium salt as a part of neutralizing salts for a water absorptive resin (Japanese Laid-Open Patent Application No. 6-25543);
9 Adding a polyamino acid (or salt) aqueous solution to a water absorptive resin, or polymerizing water soluble unsaturated monomer in the presence of a polyamino acid (or salt) and a cross link agent (Japanese Laid-Open Patent Application No. 7-310021).
According to the methods based on 7, some improvement in the blood absorption speed can be observed. The disadvantages are that the absorption efficiency and the amount of retention are lowered since a large amount of inorganic salt or organic salt must be used.
According to the methods based on 8, although the absorption efficiency for blood is improved to some extent, the level of the blood absorption speed is not satisfactory.
According to the methods based on 9, although the absorption performance for physiological saline is good, the absorption amount for blood is low, a mere 6 to 11 times the weight of the absorbent. Furthermore, the absorption speed level is unsatisfactory.
In view of the above, there exists a need for absorbent compositions, methods for producing them and absorbent products which overcome the above mentioned problems in the prior art. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.